FIG. 1 shows an exemplary propagation delay profile in a wideband radio communication system. In FIG. 1, the horizontal axis represents time (ns), and the vertical axis represents amplitude.
As seen in FIG. 1, multipaths with higher levels of reception power are observed in the range from 50 ns to 60 ns. These multipaths with higher reception levels interfere with each other, which may cause lower accuracy of channel estimation and data demodulation.
A preferred coding of pilot channels is described with reference to FIG. 2.
For temporal areas, if the auto-correlation of received pilot signals is derived, it is desired that peak codes be detected in synchronization with respective timings and non-peak codes be detected out of synchronization. Such codes may be codes with auto-correlation characteristics, for example.
Also, for frequency areas, it is desirable that the auto-correlation characteristics are flat. In other words, supposing that the horizontal axis represents frequency and the vertical axis represents auto-correlation values, it is desirable that functions with constant auto-correlation values be used. On the contrary, if the auto-correlation value may vary considerably with respect to the frequency, some influence may occur due to noise or other factors. In other words, there are two frequency areas; a higher channel estimation accuracy is achieved in one frequency area, and a lower channel estimation accuracy is achieved in the other frequency area. In the frequency area with the lower channel estimation accuracy, some influence may occur due to noise, interference or other factors.
Some cases of code evaluation in the temporal areas are described below.
In general, Merit Factor (MF) of Golay is used as evaluation values for evaluating codes.
The MF is described below.
A complex sequence u is defined as u=(u0, u1, uN-1) wherein N is the length (or the cycle) of the sequence u. Here, the auto-correlation function (ACF) of the sequence u is defined by the formula (1);
                                                        C              ^                                      u              ,              u                                ⁡                      (            τ            )                          =                              ∑                          t              =              0                                      N              -              1              -              τ                                ⁢                                    u              ⁡                              (                t                )                                      ⁢            u            *                                          (                                  t                  +                  τ                                )                            .                                                          (        1        )            
In this auto-correlation function, for a certain sequence u, the complex conjugate values are computed for the sequence u time-sifted by τ. In such an auto-correlation function, it is desirable that peaks be detected in τ=0 and no peak exist in greater shifts.
Also, MF for the sequence u is defined by the formula (2);
                    MF        =                                                                                                  C                    ^                                                        u                    ,                    u                                                  ⁡                                  (                  0                  )                                            2                                      2              ⁢                                                ∑                                      τ                    =                    1                                                        N                    -                    1                                                  ⁢                                                                                                    C                        ^                                                                    u                        ,                        u                                                              ⁡                                          (                      τ                      )                                                        2                                                              =                                                    N                2                                            2                ⁢                                                      ∑                                          τ                      =                      1                                                              N                      -                      1                                                        ⁢                                                                                                              C                          ^                                                                          u                          ,                          u                                                                    ⁡                                              (                        τ                        )                                                              2                                                                        .                                              (        2        )            
The MF may be one of standard evaluation criteria for evaluating characteristics of codes. The above function computes the sum of auto-correlation values for propagation delay amounts. In this case, the auto-correlation values for the propagation delay amounts are computed with uniform weighting.
Non patent document 1: D. C. Chu “Polyphase Codes With Good Periodic Correlation Properties,” IEEE Trans. Inform. Theory, vol. 18, no. 4, pp. 531-532, July 1972.
Non patent document 2: Cohen, M. N., Fox, M. R., and Banden, M., “Minimum peak sidelobe pulse compression codes,” Proc. IEEE Int. Radar Conf., 1990, pp. 633-638.
Non patent document 34; IEICE Technical Report vol. 105, RCS2005-215, pp. 223-227.